Process for the recovery of molybdenum and rhenium from sulfides by electrolytic dissolution

ABSTRACT

A pollution-free process for the dissolution in an acidic media of molybdenum sulfides with the formation of soluble molybdenum and rhenium ions followed by the recovery of these ions from solution in the electrolyte media, the process characterized by certain critical conditions, these being the use of: 1. AN ALKALI METAL AND/OR ALKALINE EARTH METAL CHLORIDE ELECTROLYTE; 2. AN ACIDITY RANGE OF ABOUT 10 PERCENT HCl to pH 2.5; 3. an electrolyte temperature range of about 50* C to 105* C; and 4. AN ANODE CURRENT DENSITY UP TO ABOUT 500 AMPERES/FT2.

United States Patent ['19]- Kruesi PROCESS FOR THE RECOVERY OF MOLYBDENUM AND RHENIUM FROM SULFIDES BY ELECTROLYTIC DISSOLUTION [76] Inventor: Paul R. Kruesi, 14255 Braun Rd.,

Golden, Colo. 80401 22 Filed: Mar. 2, 1971 2i Appl.No.: 120,339

OTHER PUBLICATIONS Modern Electroplating, 2nd Ed. by Lowenheim, pp.

435-437, pub. by John Wiley & Sons, Inc., New York,

Primary Examiner-F. C. Edmundson Attorney-Sheridan, Ross 8: Burton [57] ABSTRACT A pollution-free process for the dissolution in an acidic media of molybdenum sulfides with the formation of soluble molybdenum and rhenium ions followed by the recovery of these ions from solution in the electrolyte media, the process characterized by certain critical conditions, these being the use of:

1. an alkali metal and/or alkaline earth metal chloride electrolyte; v 2. an acidity range of about 10 percent HCl to pH 2.5; 3. an electrolyte temperature range of about 50 C to 105 C; and 4. an anode current density up to about 500 amperes/ft 5 Claims, No Drawings PROCESS FOR THE RECOVERY OF MOLYBDENUM AND RHENIUM FROM SULFIDES BY ELECTROLYTIC DISSOLUTION BACKGROUND OF THE INVENTION.

There are disclosures in the prior art of processes for the electrolytic recovery of certain metals from: their sulfide ores under various conditions. These processes cannot be used for the economic recovery of molybdenum and rhenium from molybdenite and other molybdenum sulfides, particularly, when the sulfide concentrate is of'low grade, for various reasons. The invention is illustrated by. itsapplicationto'molybdenite; however, it is equally applicableto other sulfide ores of molybdenum.

Molybdenite is moreresistantto attack than the base metal or nickel sulfides and as a result it-has resisted efforts to economically recoverit by electrolytic dissolution. In the Soviet Journal of Non-Ferrous Metals (Au-- vestigation Rl-6246 1963 can be applied to the recov-.

cry of molybdenum and rhenium.

Further, as pointed out in the Russian-Journal cited above, sulfate ions build up in the electrolyte and must be removed if the electrolyte is to be recycled. The entire electrolyte must be acidified to do thiswith' cal cium, as is most economic, and then in the caseof an alkaline media the media mustagain be made basic. The costs of this are obvious. In an acidic media the necessary sulfate removal with calcium is straightforward and produces readily disposed ofgypsum thereby avoiding any pollution.

Further, such potentially valuable co-products as copper,'silver and lead which often occur in molybdenite concentrates are not soluble in an alkaline (soda ash) media and so must be separately recovered. lnan acidic media, after extraction of molybdenum and rhenium, the anolyte can be sent to the cathode where these metals are recovered.

Despite some advantages of the acidic-media, prior investigators have not found it suitable because of a belief that high electrical efficiency could not be obtained in an acid environment. Thus, in the Soviet Journal referred to above the authors state that in a strongly alkaline solution -12 grams Sodium Carbonate per liter) 99 percent current efficiency can be obtained but that in a neutral solution 82 percent is a maximum,

Prior to the present time there has been little incentive for the development to commercial application of electrolytic processes for the recovery of metals from sulfide ores. Molybdenum is conventionally recovered from molybdenite by a roasting process in which the sulfur contained in the concentrate is oxidized to sulfur dioxide and discharged to the atmosphere with consequent damage to the environment. Recently promulgated pollution standards make the roasting process as presently applied prohibitive and have created a demand for pollution-free processes. An

electrolytic process which'converts the sulfur to sulfate or other form readily disposed of and which does not require prohibitive amounts of power is an answer to the pollution problem. As is well known in the art the sulfate can be disposed of without pollution problems byaddition of lime to the sulfate containing electrolyte. In the roasting process no purification of the molybdenite (except the removal of sulfur) occurs and as a result stringent specifications as ,to impurities in' the molybdenite concentrates are applied. Such potentially valuable co-products' as copper and lead'are considered harmful impurities, as concentrates containing these elements arecustomarily penalizedi In contrast, these elements can beeconomically recovered in the acidic electrolytic process of this invention and instead of being subject to penalty provide additional economic benefit.

Further, because purification does not occur in the roasting process it is applied only to high gradeproducts, and concentrates containing large amounts'of impurities'are either heavily penalized or not processed. Such concentrates can be economically processed by the present acidic electrolytic process.

Rhenium frequently occurs in molybdenite insmall percentages and is a potentially valuable co-product. In

the conventional roasting process rhenium is volatilized and recovered from the voluminousoff-gas.streams by various techniques. Recoveries are low and recovery techniques are expensive. In the acidic electrolytic pro-' cess of this invention rhenium quantitatively dissolves with the molybdenum and is readily and inexpensively recovered.

STATEMENT OF THE INVENTION The invention is apollution-free process forthe re covery of molybdenum and rhenium from sulfidej ore concentrates in which the sulfide is electrolytically'dis sociated into soluble metal ions, elemental sulfur andsulfate, the valuable metal ions being recoveredfr om.

the electrolyte by pollution free techniques knownfih the art. The electrolytic process is characterized by certain critical process conditions to render it economi cally feasible, these being the use of l an alkali metal and/or alkaline earth metal chloride electrolyte, (2,) an acidity range for the electrolyte media of about 10 percent to pH 2.5, (3) an electrolyte temperature range of about 50 C C, and (4) an anode current density up to about 500 amperes/ft.

The economic feasibility of the process is dependent upon the small amount of power required to produce a given quantity of metal ion, and the ease of recoyer ing the molybdenum and rhenium in commercially' ac} ceptable form. The amount of power required isgex-fsolution of alkali metal chloride or alkaline earth metal chloride,-or mixtures thereof. The chlorides of sodium} potassium, barium and calcium, or mixtures thereof,

have been found suitable. Concentrations withinthe' range of 0.5-4N or saturation may be used. Voltage across the cell is lower at higher salt concentrations so that the latter are preferred except where low grade feeds are used and where salt losses would therefore become significant. 5

The feed material is preferably ground to an average particle size of about -60 mesh US. Standard. A pH range between about percent HCl and 2.5 pH is used. The preferred pH is about 0.5. A temperature range of 50 C 105 C can be used with the preferred temperature being 80 C. Anode current densities up to about 500 amperes/ft can be used with 120 amperes/ft preferred for high grade material and 50 amperes/ft being preferred for low grade material.

An apparatus well known in the art was used in developing the process which consisted of an anode made of graphite or other suitably corrosion resistant material placed opposite a cathode made of stainless steel. The anode was separated from the cathode by a diaphragm of a corrosion resistant filter cloth. The anode and cathode were placed in a corrosion resistant vessel and agitation was provided in the anode compartment so that the molybdenite concentrate was circulated past the anode. Supplemental heating'was provided to controlled temperature. This type of cell may, of course, be provided with a multiplicity of anodes and cathodes.

The solution containing molybdenum and rhenium ions dissolved at the anode may conveniently be separated from the solids circulating in the anolyte by filtration or other means known to the art. The clear solution may then be subjected to solvent extraction again by means known in the art as, for example, the use of a quaternary amine as taught by Churchward (supra). Molybdenum and rhenium ions are quantitatively extracted and are readily separated one from the other by the techniques cited above. The solution containing copper, lead or other base metals may then conveniently be passed to the cathode compartment where these base metals may be reduced to metal for recovery. These techniques are again well known to the art.

The following examples performed with the above described type of electrolytic cell are presented to illustrate the invention.

In the examples, the same molybdenite concentrate was used in Examples 1, 2, 3, 5, 6 and 8 and contained 54.65 percent molybdenum, 1.13 percent copper and 0.075 percent rhenium. Anode current densities are reported in amperes/ft, current requirement is reported in ampere hours/pound of metal dissociated or recovered, current efficiency is based on theoretical, and X Cu or Re.

EXAMPLE 1 50 grams of a commercial molybdenite concentrate was subjected to 30 ampere hours of current in the electrolytic cell with the results listed below.

This example illustrates the very high efficiencies in terms of power requirement which can be obtained under the proper conditions.

EXAMPLE 2 200 grams of concentrate was subjected to 120 ampere hours of current.

ing molybdenum athigh efficiency, copper can be recovered quantitatively at the cathode and rhenium is solubilized at the same rate as molybdenum and, therefore, fully available for recovery.

EXAMPLE 3 200 grams of concentrate was subjected to 60 ampere hours of current.

even at high acidities although efficiency is somewhat lower than at lower acidities.

EXAMPLE 4 50 grams of a different commercial molybdenite concentrate assaying 53.7 percent Mo was subjected to ampere hours of current.

Media 3N KCl pH 2.0 Temperature 90% Anode Current Density 480 Soluble Molybdenum 17.4 grams Current Requirement(Mo) 2610 Current Efficiency 88% This example shows that the process is effective even at relatively high current densities.

EXAMPLE 5 44 grams commercial concentrate was subjected to a current of ampere hours.

Media 2N KCl pH 3.0 Temperature 90C Anode Current Density 360 Soluble Molybdenum 14.6 grams Current Requirement (Mo) 3720 Current Efficiency 62% This example shows the substantial decrease in efficiency when the acidity decreases to pH 3.0.

EXAMPLE 6 200 grams of concentrate was subjected to 60 ampere hours of current.

This example shows that calcium chloride can be used as an electrolyte.

EXAMPLE 7 i 400 grams of a low grade (Molybdenum 7.5 percent) molybdenite tailings was subjected to 120 aihpere hours.

Media 4N NaCl pH 0.1 Temperature 80C Anode Current Density 47 i Soluble Molybdenum 16.6 grams Current Requirement (Mo) 3360 Current Efficiency 68% sulfide ores-Recovery of valuablerhenium-in the pro-' .cess enhances its commercial attractiveness. The cost '.0f the recovery of molybdenum and rhenium from the electrolyte after electrolysis by conventional techniques is comparatively small. The. elimination of theproduction of sulfur dioxide substantially reduces the pollution problems associated with prior art processes.

Accordingly. the invention provides a process for recovery of molybdenum. from its sulfide ores which has the advantages of being commercially feasible and pollution free;

I the group consistingof molybdenum and rhenium from This example shows that the process is effective on very poor quality molybdenite concentrate.

EXAMPLE 8 100 grams of concentrate was subjected to 6 0 amperes hours of current.

Media 4N NaCl pH i 1.5 Temperature 80C Anode Current Density 240 Soluble Molybdenum 10.4 grams Current Requirement (Mo) 2605 Current Efficiency 88% The example shows that efficiency is high in the presence of barium chloride which precipitates as barium sulfate-as sulfate is formed in the electrolysis. Barium chloride is effective as an electrolyte.

The power requirements'set forth in the examples are well within commercial feasibility ranges for large scale production of molybdenum from its sulfide and mixed its sulfides and mixed sulfides which comprises:

a. providing an electrolyte in an electrolytic cell ineluding-at least ananode and a cathode, the electrolyte comprising an acidic aqueous solution of at -leastonechloride salt selected from the group consisting of alkali metal chlorides and alkaline earth "metal chlorides;

b. mixing with the electrolyte a solid feed sulfide of c. maintaining 'the temperature of theelectrolyte media at about 50' to C and the pH of the electrolyte media between about that of a 10 percent H C] solution and 2.5 while introducingelectric current into the electrolytic cell to provide an anode current density up to about 500 amperes/ft;

and i i a I d. recovering said metal from the electrolyte.

2. The process of claim 1 in which the metal recovered is molybdenum.

3. The process of claim 1 in which the metal recovered is rhenium.

4. The process of claim 1 in which the alkali metal chlorides are sodium and potassium chlorides and the alkaline earth metal chlorides are calcium and barium chlorides. I

5. The process of claim 1 in which a-pH of about 0.5 is used. f I 

1. AN ALKALI METAL AND/OR ALKALINE EARTH METAL CHLORIDE ELECTROLYTE;
 2. AN ACIDITY RANGE OF ABOUT 10 PERCENT HC1 TO PH 2.5;
 2. The process of claim 1 in which the metal recovered is molybdenum.
 3. The process of claim 1 in which the metal recovered is rhenium.
 3. AN ELECTROLYTE TEMPERATURE RANGE OF ABOUT 50*C TO 105* C; AND
 4. AN ANODE CURRENT DENSITY UP TO ABOUT 500 AMPERES/FT2.
 4. The process of claim 1 in which the alkali metal chlorides are sodium and potassium chlorides and the alkaline earth metal chlorides are calcium and barium chlorides.
 5. The process of claim 1 in which a pH of about 0.5 is used. 